The liquid-crystal display in your laptop computer looks simple enough at first glance, but it takes a very unusual material along with some complex optics and electronics to bring a slab of glass to life.
Scientists first realized in the 1880s that rather than changing straight from solid to liquid when heated as most materials do, liquid crystals have an intermediate stage where their molecules flow like a liquid, but remain in an orderly pattern and block light like a solid. Their other interesting property is that they'll align themselves to microscopic grooves in a surface or in response to an electric current. Those two factors are why liquid-crystal displays work.
A liquid-crystal display is like a sandwich. It has a filling of liquid crystal several molecules thick trapped between two sheets of glass, each one of which has a transparent electrode and microscopic parallel grooves on the inside surface. On the outside of the glass is a polarizing filter, which blocks light waves not aligned with the polarization.
The grooves inside each sheet of glass run in the same direction as the polarized filter outside. The trick is that one sheet’s polarization and grooves are at right angles to the other sheet's. One filter would block all the light that got through the other if it weren’t for the liquid crystal in the middle.
That layer is the key. There are many layers of liquid crystals in the material sandwiched between the glass sheets. The liquid crystal molecules pressing directly against each sheet of glass naturally align themselves to follow the grooves. But since the grooves on one sheet of the glass sandwich are at right angles to those on the other sheet, each layer of molecules in between twists a certain amount due to contact with the previous layer. These twisted liquid-crystal molecules form a sort of tunnel that channels light passing from one filter to the other, allowing the light to get through.
This is where the role of electricity comes into play. Passing a small electric current through the electrodes overrides the influence of the grooves in the glass and forces the liquid-crystal molecules to untwist and align themselves in a specific direction, blocking the light. So an area of the display appears bright when the current controlling it is off and light can get through, and appears dark when the current is on.
The amount of current applied to an area of liquid crystals determines how much the molecules untwist, which in turn controls how much light is blocked. It's similar to the way that a shower tap controls how much water flows, and how much hot water is added to that flow to produce a desired temperature. In the case of an LCD panel, besides basic black and white, you can produce shades of gray by varying the current to an area of the screen. Today’s LCD displays can produce 256 different light levels using this approach.
An LCD display can also be made more readable by boosting the amount of light going through it. A very basic LCD screen, such as the kind in a digital watch, may simply have a reflector behind it. Light falls on the face of the watch and is reflected back through the liquid crystal material by this reflector. Backlit watches have a light that can be turned on to make them brighter, or readable in low light. Notebook and desktop computer displays have bright lights behind them too.
The LCDs in a watch face are relatively cheap and easy to produce, because they only have to display a limited amount of information and most do it in monochrome - shades of grey. A computer display or LCD TV, in contrast, has thousands of points, or pixels, that make up the image. To display pictures, each pixel must be controlled separately. The pixels are arranged on the LCD screen in rows and columns, a bit like a piece of graph paper but on a microscopic scale. Each pixel is the intersection of a particular row and column.
In older "passive matrix" screens, all the pixels in a row share an electrode built into the sheet of glass on one side of the display, and all the pixels in a column share an electrode on the other sheet. An integrated circuit sends current to each row-and-column combination in turn, lighting up or darkening each pixel. This is done hundreds or thousands of times a second, so that the screen can show changing images or video. The problem with this passive-matrix approach is that it’s hard to control voltage precisely, and undesirable effects occur, like "ghosting," when fast-moving objects leave trails behind them across the screen.
In newer "active-matrix" displays, each pixel has its own transistor. Each row of transistors is turned on in turn, and the correct voltage sent to each pixel in the row. Each pixel also has a tiny capacitor that stores the voltage, keeping the pixel activated until it needs to be updated again when the on-screen image changes. As a result, active-matrix displays are brighter and sharper, and tend not to show any ghosting.
The final element of modern LCDs is what makes them able to display images in full colour. Three sub-pixels for each point on the screen are added to the electronics built into the glass sheets, equipped with red, green and blue filters. They alter the colour of the light passing through that tiny part of the LCD screen. With 256 light levels available for each of the three primary colours, an LCD is capable of displaying nearly 16.8 million colours, which is why things like photographs and videos appear on the screen in lifelike shades.
